[detaro]

For the receiver: Everything that can run a dynamic website can run a webhook receiver, opening an arbitrary socket connection isn't possible in all environments (e.g. software running on shared hosting or PaaS). You'd also need to define and implement a protocol on top of said socket, whereas more or less every web developer knows what to do with HTTP POST with a JSON payload.

And for the sender, keeping many concurrent connections open can be quite a challenge. Sending Webhooks also takes resources, but at least you can easily distribute it over many machines/processes if necessary.